To say "thank you" in Vietnamese, you say: "Cảm ơn"
(pronounced: gahm uhn) This is the most common and polite way to express gratitude in Vietnamese. If you want to be more formal or polite, you can say: "Cảm ơn bạn" (thank you, friend)
or
"Cảm ơn anh/chị" (thank you, addressing a male/female respectfully). Let me know if you want to learn how to say other phrases in Vietnamese!